1. Field of the Invention
The present invention relates to a membrane tension measuring apparatus designed to cause a membrane to vibrate according to acoustic wave so as to measure tension which is applied to the membrane.
2. Description of the Related Art
A building having a membrane structure includes, for example, a baseball stadium, a gymnasium and a pavilion. In this type of building, a membrane constituting a main part of the membrane structure has flexibility when it is in a free state. Therefore, during completion of the building, a uniform tension (N/cm) is applied as an initial load to the membrane of the membrane structure, along a surface of the membrane. This allows the membrane to be maintained (retained) in a given shape and imparted with stiffness for preventing easy deformation due to a load such as wind or snow.
Meanwhile, generally, as time passes after the completion of the building, the membrane is more apt to undergo loosening in the surface direction. Due to the loosening, internal stress of the membrane caused by the tension will decrease, i.e., the tension will decrease. The decrease in tension is undesirable because it is likely to give rise to excessive slack in the membrane, which causes a problem, such as fluttering or ponding.
Thus, as a prerequisite to adequately maintaining the membrane over a long period of time, it is important to know tension applied to the membrane, at intervals of a given elapsed time after the completion of the building. Therefore, there has heretofore been known a membrane tension measuring apparatus designed to allow measurement of the tension, as disclosed in JP 2002-090238A.
According to JP 2002-090238A, the membrane tension measuring apparatus comprises a rectangular frame adapted to be abuttable on a surface of a membrane to which a uniform tension is applied, an acoustic wave generator operable to emit an acoustic wave toward a part of the membrane surrounded by the frame, a vibration detector operable to detect vibration which is induced in the part of the membrane by the emitted acoustic wave; and a processor operable to calculate the tension based on vibration data detected by the vibration detector.
When a measurement task for measuring tension in a membrane is carried out using the above membrane tension measuring apparatus, the frame is firstly placed to abut on a surface of the membrane. Subsequently, an acoustic wave is emitted from the acoustic wave generator toward a part of the membrane surrounded by the frame. Thus, the emitted acoustic wave causes the part of the membrane to vibrate, and the resulting vibration is detected by the vibration detector. Then, through the processor, a resonant (natural) vibration frequency of the part of the membrane is figured out based on vibration data detected by the vibration detector, and the tension is calculated based on the resonant vibration frequency and output. This makes it possible to know the tension and thereby adequately maintain the membrane.
Meanwhile, in the membrane tension measuring apparatus disclosed in JP 2002-090238A, the frame, the acoustic wave generator, the vibration detector and the processor are provided separately with respect to each other. Therefore, in the tension measurement task using the membrane tension measuring apparatus, a measurer is required to support the above components individually. However, the measurement task, particularly, at a high location or a location with poor footing in a building, etc., is likely to become complicated. In this respect, a need for further improvement still remains.